Stability and phase evolution of mullite in reducing atmosphere

The aim of this study is to investigate the stability of mullite and its phase evolution when heated at 1600–1650 °C in reducing atmosphere created by a carbon bed. The stoichiometric mullite (3Al2O3·2SiO2) was synthesized by reacting the proper precursors at 1400 °C in air atmosphere. Samples containing the primary mullite and graphite or carbon were prepared by pressing the mix and heated at 1600–1650 °C. Products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the stoichiometric mullite could decompose to corundum and alumina-rich mullite. During firing under reducing atmosphere, the stoichiometric mullite became richer in Al3+ cations. The gaseous SiO compound was found to be released from the sample and reacting with C to form SiC phase. Comparison of general mullite solid solution (Al4 + 2xSi2 − 2xO10 − x) with alumina-rich mullite obtained from decomposition revealed that x would take different values depending on temperature and atmosphere. The ultimate decomposition product was found to be corundum.